The present invention relates to a seat belt winding device for use in a vehicle and the like, and particularly to a seat belt winding device which, when a tension exceeding a predetermined level is applied to a seat belt due to shocks produced in a collision or the like, draws out the seat belt to lessen a deceleration acting on the body of an occupant, thereby being able to protect the occupant more effectively.
Conventionally, in a vehicle or the like, there is used a seat belt winding device employing a seat belt which restrains the body of the occupant to a seat for protecting the occupant from collision shocks. In the conventional seat belt winding device, there is employed a clamping mechanism which, in an emergency such as a collision or the like, not only locks the rotation of a winding shaft with the seat belt wound therearound but also holds the seat belt to prevent the seat belt from being drawn out, whereby it is improved in its performance to restrain the occupant to the seat in the early stages of the collision.
A conventional seat belt winding device with a simple clamping mechanism functioning so as to prevent the seat belt from drawing out by locking the webbing, operates as indicated by a dotted line in FIG. 25, in the emergency. As time passes after vehicle collision, a tension of the webbing increases. When a shock by the collision is extremely large, an abrupt deceleration acts on the body of the occupant.
To solve this problem, there is proposed an improved version of the above-mentioned clamping mechanism. According to the improved clamping mechanism, in the early stages of the collision in which the tension to be applied to the seat belt due to the collision is smaller than a predetermined value, the clamping mechanism is operated to prevent the draw-out of the seat belt in order to restrain the occupant body to the seat positively. After then, at a time when the tension acting on the seat belt exceeds a predetermined value, the operation of the clamping mechanism is removed to draw out the seat belt a predetermined amount, thereby lessening the deceleration applied to the occupant body to protect the occupant body more positively.
In FIGS. 26 and 27, there is shown a conventional seat belt winding device including a clamping mechanism which, in the early stages of a collision where the tension acting on a seat belt or a webbing due to the collision is smaller than a predetermined value, holds the webbing therebetween to prevent the draw-out of the webbing and, after the webbing is held therebetween, when the tension acting on the webbing exceeds a predetermined value, allows the draw-out of the webbing.
The seat belt winding device 1 shown in FIGS. 26 and 27 is disclosed in Unexamined Japanese Utility Model Publication No. Hei. 4-43551. According to the present seat belt winding device 1, in an emergency such as a vehicle collision or the like, a winding shaft 3 with a webbing 2 (which serves as a seat belt) wound thereon is locked by an emergency locking mechanism 4 to prevent the draw-out of the webbing 2 due to the rotational movement of the winding shaft 3. At the same time, the webbing 2 drawn out on the back plate portion 5a of a retractor base 5 from the winding shaft 3 is held by and between a clamping mechanism, thereby preventing the webbing 2 remaining wound on the winding shaft 3 (actually, on a bobbin) from being drawn out due to the tightening of the webbing 2.
The clamping mechanism includes a clamp shaft 6 having the two ends thereof supported by a pair of base side plates 5b of the retractor base 5, a clamp holder 7 supported such that it can be swung about the clamp shaft 6 along the webbing 2 drawn out on the back plate portion 5a of the retractor base 5, and a clamp member 8 mounted on the end portion of the clamp holder 7. According to this clamping mechanism, in an emergency, the clamp holder 7 is swung in a direction of an arrow B in FIG. 26 to press the clamp member 8 against the webbing 2 so that the webbing 2 is held by and between the back plate portion 5a and clamp member 8, thereby preventing the draw-out of the webbing 2 in a direction of an arrow A in FIG. 26.
Here, the clamp member 8, as also shown in FIG. 27, has a semi-cylindrical outer shape and includes in the two end portions thereof two support shaft portions 8a which can be rotatably supported by the clamp holder 7, while the clamp member 8 is rotatably held on the end portion of the clamp holder 7 with the semi-circular outer peripheral surface 8b thereof situated on the side of the clamp holder 7. On the flat surface of the clamp member 8 facing the webbing 2, as shown in FIG. 27, there are formed a large number of holding portions (clamp teeth) 8c which, when pressed against the webbing 2, stick into the webbing with their respective sharpened portions to prevent the draw-out of the webbing. Further, the strengths and dimensions of the respective holding portions 8c can be set appropriately so that, when a load of a predetermined value is applied to the end portions of the holding portions 8c, the end portions can be sheared to remove the webbing from its holding state.
According to the conventional clamping mechanism shown in FIGS. 26 and 27, in the early stages of occurrence of an emergency, the clamp member 8 is pressed against the webbing 2 to stick the holding portions 8c of the clamp member 8 into the webbing 2 in the holding state. Thus, the webbing 2 is firmly secured to prevent the draw-out of the webbing 2. After holding the webbing, when the tension acting on the webbing 2 due to the inertial energy or the like of the occupant exceeds a predetermined value, the holding portions 8c are sheared to remove the webbing from its holding condition, in order to lessen the shocks to be applied to the occupant body due to the restraint of the webbing.
Now, in FIG. 28, there is shown a webbing holding condition in which the clamping mechanism is operated and thus the clamp member 8 sticks into the webbing 2 with the holding portions 8c of the clamp member 8 to prevent the draw-out of the webbing 2. Also, in FIG. 29, there is shown a condition in which, after the webbing is held, the tension acting on the webbing exceeds a predetermined value and thus the holding portions 8c are sheared to remove the holding of the webbing. In this case, the end portions 8d of the sheared holding portions 8c are left embedded in the webbing 2.
In this manner, in the structure that the webbing holding is removed if the tension acting on the webbing after the webbing holding increases and exceeds a predetermined value, the variations of the webbing tension after the occurrence of collision show such a characteristic as indicated by a two-dot chained line in FIG. 25. That is, when the webbing tension reaches a predetermined value P1, then the draw-out of the webbing due to the tightening thereof or the like starts so that the webbing tension decreases and, if the draw-out of the webbing is prevented, then the webbing tension rises again from the then tension P2 up to a tension P3 of a value greater than the P1. According to this structure, the maximum tension acting on the seat belt can be reduced as compared with a structure employing a clamping mechanism having no clamp removal function.
The actual operation for reducing the maximum tension acting on the seat belt in this manner is not limited to the above-mentioned shearing of the holding portions 8c. For example, in U.S. Pat. No. 5,299,854, there is disclosed a technique in which a part of the side wall portion of a retractor base supporting a clamp shaft is designed as a weak portion which is low in strength. When the tension acting on the webbing increases and exceeds a predetermined value, then the webbing holding condition is removed by deforming or breaking the weak portion, thereby preventing an increase in the webbing tension.
Also, in Unexamined Japanese Utility Model Publication No. Hei. 4-43550, there is disclosed a technique which, if the tension acting on the webbing increases and exceeds a predetermined value, then a clamp shaft is plastically deformed or broken by itself to remove the webbing holding condition so as to prevent an increase in the webbing tension.
Further, in U.S. Pat. No. 5,242,213, there is disclosed a technique which, if the tension acting on the webbing increases and exceeds a predetermined value, then a retractor base itself supporting the clamping mechanism is broken in such a manner as to permit the movement of the clamping mechanism in the webbing draw-out direction, thereby preventing an increase in the belt tension.
In the above-mentioned technique disclosed in Unexamined Japanese Utility Model Publication No. Hei. 4-43551, to appropriately reduce the maximum tension acting on the seat belt, a timing for removing the webbing holding by shearing the holding portions 8c must be made to correspond accurately to the tension acting on the webbing. Also, to reduce effectively shocks to be applied to the occupant body from the webbing in a collision or the like, not only the maximum tension acting on the seat belt must be reduced but also it is important to avoid an abrupt increase or decrease in the tension of the seat belt as well as to reduce the width of variations in the tension.
However, in the structure of the above conventional clamping mechanism, the timing (webbing tension) for shearing the holding portions 8c tends to vary according to the acting conditions (deceleration conditions) of the shocks occurring in a vehicle collision or the like, which makes it very difficult to correspond the timing for removing the webbing holding to the tension acting on the webbing, with the result that the shock absorbing performance as designed cannot be obtained.
There would be considered the causes of the above problem found in the conventional clamping mechanism as follows.
For example, as shown in FIG. 30, a shearing force F.sub.1 due to the tension acting on the webbing 2 and a reaction force F.sub.2 to a pressing force to the webbing 2 due to the swinging movement of the clamp holder 7 act on the holding portions 8c of the clamp member 8 pressed against the webbing 2. Actually, it is assumed that the resultant force F.sub.3 of these two forces F.sub.1 and F.sub.2 provides a force to break the holding portions 8c. However, in the above conventional clamping mechanism, after the webbing is held, until the holding portions 8c are broken by the force F.sub.3, the pressing operation due to the swinging movement of the clamp holder 7 continues, the force F.sub.2 continues increasing according to an increase in the tension acting on the webbing 2. Further, the force F.sub.2 varies in the increasing speed in accordance with the increasing speed of the tension and the like. Due to these facts, the force F.sub.2 has a very great effect on the force F.sub.3 at the time when the holding portions 8c are actually broken.
Also, in any of the above-mentioned conventional structures, after the webbing holding by the clamping mechanism is removed, as also shown in FIG. 25, since the increase and decrease of the tension are repeated over a short time, there is a fear that the tension acting on the seat belt may be increased and decreased abruptly and the variation width thereof may be large.